1. Field of the Invention
The present invention is directed to a process for the production of stable, aqueous dispersions of polyurethane-ureas having good hydrolytic stability and containing chemically incorporated carboxylate groups which are neutralized with specific alkali metal cations.
2. Description of the Prior Art
The production of linear or cross-linked aqueous polyurethane-urea dispersions is known as shown by U.S. Pat. Nos. 3,479,310; 4,066,591; 4,092,286; 4,108,814; 4,237,264: and 4,238,378 which disclose linear polyurethane-ureas and U.S. Pat. Nos. 3,870,684, 4,203,883 and 4,408,008, which disclose cross-linked polyurethane-ureas. The aqueous polyurethane-urea dispersions may be used for a wide range of commercial applications such as adhesives or coatings for various substrates including textile fabrics, plastic, wood, glass fibers and metals. Chemical resistance, abrasion resistance, toughness, tensile strength, elasticity and durability are among the many desirable properties of these coatings. In some cases these properties of coatings prepared from aqueous polyurethane-urea dispersions have equaled or even surpassed the performance levels of coatings obtained from solvent-based polyurethane lacquers.
Most of the commercial aqueous polyurethane-urea dispersions contain chemically incorporated carboxylic acid groups which are neutralized with volatile organic bases such as tertiary amines to form the corresponding carboxylate groups. These groups provide hydrophilicity to the polyurethane-ureas and enable them to be stably dispersed in water. One of the properties of coatings produced from these aqueous polyurethane-urea dispersions which needs to be improved in order to match the performance of coatings prepared from solvent-based polyurethane lacquers is the hydrolytic stability, i.e. the resistance to degradation of coatings properties caused by humidity aging. This property is adversely affected by the volatile base-neutralized carboxylate groups.
Previous methods to improve the hydrolytic stability have been disclosed in U.S. Pat. No. 4,501,852 wherein aqueous polyurethane-urea dispersions are prepared by using a specific mixture of volatile and nonvolatile bases to neutralize the chemically incorporated acid groups. It is disclosed therein that the presence of nonvolatile cations such as sodium increases the hydrolytic stability of coatings produced from the dispersions, while the presence of cations produced from volatile bases increases the water resistance or resistance to water swell of the coatings.
U.S. Pat. No. 4,401,786 discloses the addition of electrolytes, preferably ones based on divalent cations, to existing polymer dispersions to improve the water resistance and/or blocking resistance of coatings prepared from the dispersions. However, the stability of these dispersions is affected by the addition of electrolytes unless they also contain nonionic hydrophilic groups. In addition, the use of electrolytes based on divalent cations appears to be necessary when the polymer dispersions are based on polyurethane-ureas.
U.S. Pat. No. 4,385,137 also discloses improving the water resistance of existing aqueous dispersions containing chemically incorporated sulfonate groups by treating the dispersions with a cation exchange resin to replace the alkali metal or ammonium cations initially present with silver, monovalent copper or divalent cations. Finally U.S. Pat. No. 4,293,474 discloses aqueous polyurethane-urea dispersions containing both ionic and nonionic hydrophilic groups in admixture with electrolytes and U.S. Pat. No. 4,171,491 discloses the coagulation of aqueous polyurethane dispersions by treatment with electrolytes.
Even though methods are available for improving the hydrolytic stability or water resistance of aqueous polyurethane-urea dispersions there is still a need for a simple method of improving the hydrolytic stability of existing, fully prepared dispersions.
Accordingly, it is an object of the present invention to provide aqueous polyurethane-urea dispersions which may be used to prepare coatings having improved hydrolytic stability.
It is an additional object of the present invention to provide aqueous polyurethane-urea dispersions which may be used to prepare coatings having excellent chemical resistance, abrasion resistance, toughness, tensile strength, resistance to water swell, elasticity and durability.
It is a further object of the present invention to be able to post treat existing aqueous polyurethane-urea-dispersions which may then be used to prepare coatings having improved hydrolytic stability.
Surprisingly, it has been found that these objectives may be achieved according to the present invention as hereinafter described.